1. Field of Invention:
This invention relates generally to pressure gauges, and more particularly to an accurate, direct-action pressure sensor usable in the low pressure range and including a pressure-responsive bladder made of elastomeric material reinforced by a helical coil which does not impede flexing of the bladder but prevents dilation thereof, thereby maintaining a proportional relationship between the applied pressure and the indication thereof.
2. Status of Prior Art:
Measurement of absolute pressure, gauge pressure, vacuum and draft pressure, and differential pressure, is carried out by two primary types of pressure-sensitive elements; the first being the liquid column in which the height and density of the liquid are utilized to measure pressure; the second being the elastic pressure device. The sole concern of the present invention is with elastic pressure elements which are designed to follow the physical law that within an elastic limit, stress is proportional to strain; hence deflection is proportional to applied pressure.
The Bourdon tube, because of its stability, simplicity and high pointer torque, is widely used as a pressure or vacuum indicator. The operation of the Bourdon tube is based on the principle that an elastic tube having an internal cross-section that is not a perfect circle, if bent or distorted, has the property of changing its shape with internal pressure variations. This internal pressure causes the cross-sectional form to become more circular, giving rise to a motion of the closed end or tip of the tube if the open end is rigidly fixed, this motion being usually referred to as "tip travel."
While Bourdon tubes are capable of operating within various pressure ranges running as high as 100,000 psi, in no instance is it possible, as a practical matter, to operate such tubes below 12 or 15 psi. Hence, despire the advantages of Bourdon tubes, they are not effective as gauges in the low-pressure range, and it has heretofore been necessary to employ costly and relatively fragile bellows or diaphragm-sensing elements to effect measurement in this range.
My prior U.S. Pat. Nos. 3,603,153; 3,732,733 and 4,015,478 disclose a low-cost pressure sensor capable of accurately gauging pressure in the low pressure range. The sensors disclosed in these patents include two curved flat metal springs in a concentric arrangement wherein the springs are cantilevered from a socket, the free end of the springs being joined together at a tip from which a pointer extends. The springs enclose a flexible bladder having an internal chamber which communicates with the socket, whereby fluid fed through the socket into this chamber acts to expand the bladder and causes the springs to uncurl, thereby moving the tip and the pointer attached thereto as a function of the applied pressure. Also of prior art interest is my U.S. Pat. No. 3,486,400.
In a conventional Bourdon tube pressure gauge, the relationship between applied fluid pressure and tip travel is such that it takes a large pressure change to effect a relatively small tip movement. Consequently, commercial forms of such gauges include gear works or linkages to mechanically magnify the tip travel. This friction introduced by mechanical amplifiers usually makes it necessary to tap the gauge to cause it to register. Such tapping is unnecessary with a sensor of the type disclosed in my prior patents, for no mechanical amplification is entailed. The sensor is highly sensitive so that a small change in applied pressure gives rise to a relatively large excursion of the tip.
The bladder disclosed in my prior patents is made by uniformly coating a fabric sleeve formed from woven, synthetic fibers such as polyester "DACRON" fibers, with silicone rubber. To create the bladder, one end of the reinforced sleeve is sealed, whereas the other end remains open and communicates with a pressure input socket.
Ideally, a bladder of this type should be highly flexible but non-dilatable so that the internal surface area of the pressure chamber created by the bladder is constant and unaffected by fluid pressure. In this ideal form, the effect of fluid pressure on the bladder sandwiched between the C-shaped inner and outer springs is to more or less straighten out the bladder against the tension of these springs, in which case a proportional relationship will exist between the degree of applied pressure and the indication thereof that is determined by the extent to which the bladder straightens out.
But with a bladder formed by an elastomer-coated woven sleeve, this sleeve is not entirely effective in resisting dilation of the bladder. The reason for this is that the intersecting warp and woof threads of the sleeve, when subjected to internal pressure, are caused to change their angular relationship, and some dilation of the sleeve results from this action. This dilation adversely affects the accuracy of the sensor.
Also an elastomeric bladder reinforced by a fabric sleeve is not as flexible as one that is free of such woven reinforcement, for the weave of the sleeve somewhat resists flexure and this impairs the sensitivity of the sensor and introduces hysteresis.